In trunk-line optical communication systems, large-capacity optical transmission is performed using a wavelength division multiplexing communications system. Multiple optical signals of different wavelengths are transmitted through a single fiber in this system. In such a wavelength division multiplexing communications system, a tunable laser capable of varying an oscillation wavelength within a broad wavelength range is indispensable. In the wavelength division multiplexing communications, predetermined wavelength channels (an ITU-T grid) are set, and the wavelength of each tunable laser is tuned to each wavelength channel.
The transmission capacity of a wavelength division multiplexing communications system is the product of a bit rate per wavelength channel and the number of wavelength channels, and the transmission capacity increases as the number of wavelength channels increases. The number of wavelength channels is determined by an employed wavelength range (for example, a wavelength range of 1525 nm to 1565 nm referred to as “C-band”) and the wavelength spacing (interval) of wavelength channels. Accordingly, even in the same wavelength range, it is possible to increase the number of wavelength channels and accordingly to increase the transmission capacity by reducing the wavelength spacing.
In the present wavelength division multiplexing communications systems, the modulation baud rate of each wavelength is 10 Gbaud or 25 Gbaud, and as depicted in FIG. 1A, the wavelength spacing is set to 50 GHz (approximately 0.4 nm). In contrast, in next-generation wavelength division multiplexing communications systems, it has been under study to increase the transmission capacity by reducing the wavelength interval to the same value as the modulation baud rate, which is a physical limit, using the Nyquist method or optical orthogonal frequency-division multiplexing (optical OFDM). Specifically, as depicted in FIG. 1B, it has been under study to increase the transmission capacity by reducing the wavelength interval to the same value as the modulation baud rate, which is a physical limit, namely, 25 GHz in the case of 25 Gbaud.
Reference may be made to Japanese Laid-open Patent Publication No. 2006-245344 for related art.